The present invention relates generally to improved circuits and methods for generating the gamma correction voltages required for achieving satisfactory performance in driving LCD displays (liquid crystal displays), and more particularly to circuits and methods which allow more efficient optimization of gamma correction voltages needed to provide suitable images on the LCD displays. The invention also relates to improved circuits and methods which allow improved dynamic gamma voltage correction.
Color LCD displays are widely used for desktop computers and laptop computers, and consist of LCD pixel elements that are typically controlled by a matrix of intersecting gate drivers (also known as row drivers) and source drivers (also known as column drivers). Referring to “prior art” FIG. 1, the source drivers in source driver switch circuitry 18 are used to control the gray scale of each pixel by converting the digital image data 36 into corresponding voltages produced by means of a resistor-string DAC 23 and multiplexing the appropriate voltages by means of the source driver switch circuitry 18 to appropriate outputs 20-1,2 . . . q coupled to corresponding columns of pixel elements. The transmission characteristic of resistor-string DAC 22 is typically “nonlinear” to compensate for the non-linear transmission characteristic of the LCD display 11. The nonlinear behavior of resistor-string DAC 22 can be thought of as being represented by an “intrinsic” gamma correction curve (sometimes also referred to as a “color curve”). The nonlinear transfer function of each LCD display 11 is unique, and therefore the intrinsic gamma curve built into the source driver circuitry 16 by resistor-string DAC 22 has to be modified to achieve optimum display performance. (See U.S. Pat. No. 5,572,211 entitled “Integrated Circuit for Driving Liquid Crystal Display Using Multi-Level D/A Converter” issued Nov. 5, 1996 to Erhart et al., which is incorporated herein by reference.)
Source driver switch circuitry 18 and “resistor-string” DAC 22 are included in a source driver circuit 16, the outputs of which are produced on conductors 20-1,2 . . . q, where q is the number of columns of pixel elements in LCD display 11. q may be very large, for example 4096, for a very wide LCD screen 11. The resistors 23 in source driver resistor-string DAC 22 are connected in series between a high reference voltage VH and a low reference voltage VL, and the voltages at the junctions between conductors 19-1,2 . . . m define an “intrinsic” gamma curve. (As an example, the number of resistors is m=256 for an 8-bit source driver.) This intrinsic gamma curve is often adjusted for optimal panel performance by means of an external high-precision resistive voltage divider 13 including n precision resistors R1, R2 . . . Rn that also are coupled in series between VH and VL. VH, VL, and the various junctions between precision resistors R1, R2 . . . Rn are coupled either directly to conductors 19-1,2 . . . m, respectively, or are coupled to the inputs of buffers 2-1,2 . . . m as shown in FIG. 1. The outputs of buffers 2-1, 2 . . . m are connected to conductors 19-1,2 . . . m, respectively (where m=n−1). The values of precision resistors R1,2 . . . n usually are painstakingly determined (in the manner subsequently described) in order to optimize the display gamma curve by externally modifying the intrinsic gamma curve established by resistor-string DAC 22 for best display viewing performance. That approach is costly because the required calculations and trial-and-error experimentation required to obtain the resistor values is subjective, difficult, and time-consuming.
Alternatively, changes can be made in the integrated circuit mask used to manufacture source driver circuitry 16 in order to provide precise adjustments to the values of the various resistors 23 so as to obtain the desired gamma curve. However, that approach usually has been found to be too difficult and costly, because it would require adjustment for each LCD panel, as every LCD panel is different, and there are lot-to-lot differences resulting from manufacturing variations.
The “gamma voltage correction” involves correcting the above-mentioned intrinsic gamma curve so as to make the “gray scale” of displayed LCD screen images appear more satisfactorily in the eyes of a trained expert. FIG. 3 shows a typical LCD display intrinsic gamma curve, wherein the gray scale of LCD pixels is plotted versus the digital codes representing the image data applied via conductors 20-1,2 . . . q to pixels in the selected rows of TFT-LCD display panel 11 in FIGS. 1 and 2. The digital codes GMA 1-m correspond to the conductors 19-1,2 . . . m in FIG. 1 and represent the gamma correction input voltages provided to source driver circuitry 16. The intrinsic gamma curve is adjusted for better panel performance by “forcing” GMA nodes 19-1,2 . . . m to specific voltage levels.
In the prior art, one technique for generation of an intrinsic gamma curve for a particular LCD screen involves a subjective, time-consuming optimization of the values of precision resistors R1,2 . . . n in an external resistive voltage divider string to produce the correct gamma correction voltages at the various nodes of a resistive voltage divider which constitutes resistor-string DAC 22. The resistor values determined during the optimizing process are utilized to manufacture resistive voltage dividers for the LCD TV displays. The various nodes of the resistive voltage divider typically are connected to corresponding nodes of the resistor-string DAC 22 and to inputs of buffer circuits 2-1,2 . . . m, the outputs of which drive source driver switch circuitry 18 of a conventional TFT-LCD panel (thin-film transistor LCD panel). The gamma correction buffers for TFT-LCD panels must be set to appropriate voltages so that the desired gamma curve is accurately represented by the range of gamma correction voltages produced by the various buffers.
This technique of optimizing values of precision resistors R1,2 . . . n in the resistive voltage divider is very time-consuming, because a person expert in adjusting gamma correction voltages so as to produce images of desirable quality must be involved in the trial-and-error selection of precision resistors utilized in the resistive voltage divider. The procedure can require many hours to determine the values of all of the resistors of the resistive voltage divider. In some cases precision potentiometers can be utilized to optimize the resistors of the voltage divider, but the “programming” nevertheless is very time-consuming. In any case, the optimum values of the resistors R1,2 . . . n of the external resistive voltage divider then must be used in assembling identical resistive voltage dividers in each gamma reference voltage generator to produce the correct gamma correction voltages to be provided as inputs to each of the source driver circuits. This procedure must be repeated for each different kind of TFT-LCD display. The precision resistors are expensive, and the assembly of the resistive voltage divider of optimally selected precision resistors also is expensive.
Present gamma correction schemes like the one shown in prior art FIG. 1 for resistor-string DACs are not inherently limited in the number of “DAC channels”, i.e., channels of gamma reference voltage correction. For example, there are LCD displays presently available that use up to 22 channels of gamma reference voltage correction. However the higher the number of channels of gamma reference voltage correction, the more difficult and time consuming the optimization process becomes.
Furthermore, the above described prior “manual” programming technique cannot be used if “dynamic gamma voltage correction” is desired to provide dynamic or real-time improvement of picture quality in LCD panels or to adjust for variations in temperature or ambient light conditions. A single DAC having an output multiplexed to multiple sample-hold circuits which store the needed gamma correction voltages has been used in conjunction with dynamic gamma correction, wherein the sample-hold circuits repetitively refreshed during the raster scanning process.
FIG. 2 shows a TFT-LCD display system 1B in which TFT-LCD display panel 11, gate driver circuitry 12, controller circuitry 32, and source driver circuitry 16 are generally the same as in FIG. 1. However, the inputs of buffers 2-1,2 . . . m are connected to the outputs of m corresponding sample/hold circuits 5-1,2 . . . m as shown, instead of being connected to the various junctions of an external resistive voltage divider 13 as shown in FIG. 1. The inputs of the various sample/hold circuits 5-1,2 . . . m are coupled by corresponding conductors 9-1,2 . . . m, respectively, to the outputs of a single multiplexer 6. The output of a single DAC 7 is connected to the input of multiplexer 6. The digital input of DAC 7 is generated by a control interface logic circuit 8, the output of which is controlled in response to signals 34 produced by controller circuitry 32, wherein controller 32 retrieves the stored data from an EEPROM 26 for one or multiple gamma curves and accordingly updates DAC registers (not shown) that are included in control interface logic 8.
Thus, a single DAC 7 combined with a multiplexer and multiple sample/hold circuits 5-1,2 . . . m have been used to provide the required gamma correction voltages. The circuitry including control interface logic 8, DAC 7, multiplexer 6, and sample/hold circuits 5 is well known, as it is used in various TFT-LCD reference voltage generator products produced under the trademark ELANTEC by Intersil America, Inc.
To determine the values of the digital DAC inputs in FIG. 2 which represent an optimized initial static gamma curve, the values of the digital inputs to the DAC could be adjusted under the control of an expert who is highly skilled in visualizing and correcting displays on LCD screens. The expert could adjust the DAC output values so as to adjust the gamma voltages to values that produce a gray scale that is satisfactory to the expert. Those digital input values to the DAC then could be stored in a suitable non-volatile memory, such as EEPROM 26.
However, it is believed that no one has yet been successful in fully or substantially automating the initial generation of the static gamma curve in an LCD display system. (Usually, such generation of the static gamma curve is performed only once or twice during the life of an LCD display.)
Thus, there is an unmet need for a system and method which avoids the need for repetitively refreshing the sample-hold circuits used in some prior art gamma correction voltage systems.
There also is an unmet need for a system and method that both allows fast programming and fast updating of all “gamma channels” for dynamic gamma control in an LCD display system.
There also is an unmet need for a system and method which avoids costs of maintaining an inventory of precision resistors for resistive voltage dividers required in some prior art gamma correction voltage systems.
There also is an unmet need for a system and method for more effectively and more rapidly accomplishing dynamic gamma voltage correction of a TFT-LCD display panel.
There also is an unmet need for an economical way of providing a larger number of accurate gamma voltages to more accurately represent color curves for TFT-LCD display panels.
There also is an unmet need for a gamma reference voltage generating system which will make it more practical to automate the initial generation of the static gamma curve in an LCD display system.